1. Field of the Invention
The present invention relates to a sensing device which is provided with an adsorption layer for adsorbing a sensing target on an electrode surface of a piezoelectric resonator such as a quartz resonator, and detects and measures the sensing target based on variation in natural frequency of the quartz resonator caused by adsorption of the sensing target to the adsorption layer, especially to a Colpitts oscillation circuit formed with a quartz resonator and a transistor amplifier circuit.
2. Description of the Related Art
This kind of the sensing devices is commercially practical as a device which, for instance, combines a substance to be measured with the antibody to make an immune reaction with the substance to be measured, utilizing an antigen-antibody reaction as an immune reaction, and detects and measures the substance to be measured based on the degree of the reaction.
FIG. 10 shows a basic structure of a sensing device. An piezoelectric resonator, for instance, a quartz resonator 100 is structured by forming electrodes 102 and 103 on both surfaces of a quartz substrate 101, and is fixed in a receptacle 104. The electrodes 102 and 103 are connected to an oscillation circuit 107 through lead wires 105 and 106, and the oscillation circuit 107 oscillates at the natural frequency of the quartz resonator 100. The oscillation output of the oscillation circuit 107 is detected by a frequency counter 108 as a frequency signal, and the frequency of the frequency signal is displayed on a display 109. The antibody is applied to the surface of the electrode 102 which is one of the electrodes of the quartz resonator. The amount of adsorption of the substance to be measured on the antibody is measured as change in oscillation frequency of the oscillation circuit 107 by bringing the electrode 102 into contact with a solution containing the substance to be measured (refer to, for instance, Patent Document 1).
Here, the oscillation circuit 107 using the quartz resonator is structured as a Colpitts oscillation circuit or a Hartley oscillation circuit which can generally perform oscillation operation with a quartz resonator and an amplifier. As the amplifier, an analogue amplifier provided with a feedback circuit between an output and an input of an inverting logic element, or a transistor amplifier circuit with a transistor and a capacitor or a resistor is used.
[Patent Document 1] Japanese Patent Application Laid-open No. 2001-83154